1. Field of the Invention
The present invention relates to a composite substrate containing a nanocarbon material which is expected to be applicable to a reinforcing material, an electron-emitting element material, an electrode material for batteries, an electromagnetic wave-absorbing material, a catalytic material, an optical material, etc.
2. Description of the Related Art
Since the nanocarbon material has a microstructure of the order of nanometers in size and is constituted by carbon atoms of an sp2 hybridized orbit, the nanocarbon material can exhibit unique characteristics that is superior to those of conventional carbon materials and that are unknown in conventional carbon materials. Therefore, the nanocarbon material is expected to be applicable, as a high-performance next-generation material, to a reinforcing material, an electron-emitting element material, an electrode material of a battery, an electromagnetic wave-absorbing material, a catalytic material, an optical material, etc.
With regard to the method of synthesizing the nanocarbon material such as carbon nanotube, there are known several methods including an arc discharging method, a laser ablation method, a plasma chemical vapor deposition method, a thermochemical vapor deposition method, etc. Among these methods, since the arc discharging method, the laser abrasion method and the plasma chemical vapor deposition method are executed through the utilization of a non-equilibrium reaction, various problems occur, as is generally known. Namely, amorphous components are more likely to be produced, the carbon nanotube yield is low, and the thickness and kinds (configuration) of carbon nanotube to be created are non-uniform.
Meanwhile, JP-A 2002-255519 and JP-A 2002-285334 disclose a thermochemical vapor deposition method for manufacturing carbon nanotube wherein a hydrocarbon gas is thermally decomposed in the presence of a catalyst. The thermochemical vapor deposition method is known to exhibit a relatively high yield because of the utilization of a chemical equilibrium reaction. According to this method, it is possible to obtain carbon fibers which can be grown using, as a core, catalytic particles such as ultra-fine iron or nickel particles. The carbon fibers obtained from this method are formed of a carbon network layer which is grown concentrically or as having a hollow configuration.
This method however is accompanied with problems that it is difficult to control the particle diameter or chemical state of metals to be employed as a catalyst and that it is impossible to synthesize a nanocarbon material which is controlled in configuration and in thickness. Namely, it is impossible to optionally obtain a nanocarbon material having a desired structure demanded in a specific actual use, resulting in that it is impossible to avoid deterioration of yield.
Further, in the case of the conventional nanocarbon materials, it is required, on working the nanocarbon material into electrodes for batteries for example, to refine a reaction product containing carbonaceous impurities other than nanocarbon material such as graphite particles and amorphous carbon in order to selectively extract the nanocarbon material out of the reaction product. Moreover, it is required to scrape off the nanocarbon material that has been grown on a substrate in order to collect a required quantity of carbon nanotube. For these reasons, it has been impossible to manufacture desired parts such as electrodes, etc., at low costs by making use of a large quantity of a nanocarbon material having a desired configuration.
Additionally, although the conventional nanocarbon materials are fibrous in structure each fiber exhibiting crystallinity, the aggregate thereof in gram quantities is a disordered aggregate mass and formed of a powder-like or cluster-like solid having a low density. Even if the nanocarbon material of this kind is turned into a paste or mixed with other kinds of material such as a resin, etc., in order to make it useful as a practical material, it has been difficult to obtain a uniform mixture because of the fact that the aggregate of the nanocarbon materials is a disordered mass having a low density.
On the other hand, a method of synthesis which makes it possible to synthesize a high-purity carbon nanotube at a high yield without requiring refining is disclosed in JP-A 2003-12312. This method is based on a peculiar interface decomposition reaction that can be brought about through a contact between a solid substrate and an organic liquid under large temperature difference. This method is called “a solid/liquid interface contact decomposition method”. Followings are rough explanation of this method.
At first, a thin film of a transition metal such as Fe, Co, Ni, etc., is deposited on an electrically conductive silicon substrate. Then, this substrate is exposed to a hydrogen plasma or is heated so as to thermally oxidize the thin transition metal film, thereby enabling the substrate to carry catalytic fine particles distributed at a high density. Thereafter, the resultant substrate is immersed in an organic liquid such as methanol and then heated through the application of an electric current to the substrate. As a result, the substrate and the organic liquid can contact each other under large temperature difference, thereby allowing a peculiar interface decomposition reaction to take place, thus synthesizing carbon nanotube on the surface of catalytic fine particles. According to this method, it is possible to deposit, on the surface of the substrate, a nanocarbon material which is high in density, high in purity and low in manufacturing cost.
Depending on the end-use of the nanocarbon material, there is a situation where the nanocarbon material may preferably be formed at a higher density and with a larger surface area. For example, in the case of an electrode material for batteries, the storage efficiency or power efficiency can be enhanced by increasing the surface area of the nanocarbon material. Further, in the case of a field-emission-type electron-emitting element, the field concentration efficiency can be enhanced by increasing the aspect ratio of a nanocarbon material or of a substrate provided with a nanocarbon material, thereby enabling electrons to emit at a lower voltage. However, when the aspect ratio of a substrate is increased, it is difficult to enable a raw material to reach a deep inner portion of the substrate. As a result, it has been difficult to deposit a uniform nanocarbon material in a structure exhibiting a large aspect ratio. Therefore, no one has succeeded as yet to obtain such a structure that is suited for use in the above-described end-uses, that is formed of highly densified nanocarbon material, and that makes it possible to utilize a large surface area and a high aspect ratio.
The following are explanation of the problems accompanying the field-emission-type electron-emitting element. In the case of an electron display device, one having an array of minute electron-emitting elements, especially field-emission-type electron-emitting elements, which are disposed in a high-vacuum plane cell is regarded as promising. In this field-emission-type electron-emitting element, the following phenomenon is utilized. Namely, as the intensity of the electric field to be applied to a substance is increased, the width of the energy barrier of the surface of substance is gradually narrowed in conformity with the electric field intensity and when the electric field intensity is increased to 107 V/cm or above, the electrons in the substance can break through the energy barrier because of the tunnel effect, to be thereby emitted from the substance. In this case, since the electric field varies in accordance with Poisson's equation, it is possible for cold electrons to effectively be emitted at a relatively low extraction voltage when an electric field concentrating portion is formed in an electron-emitting member, i.e., emitter.
In recent years, the nanocarbon material has been noticed as an emitter material. A carbon nanotube which is the most representative nanocarbon material, is formed of a hollow cylindrical body of Grapheme sheet constituted by regular arrays of carbon atoms, the cylindrical body having an outer diameter of the order of nanometers and a length ranging from 0.5 micrometers to several tens of micrometers, thus representing a minute substance exhibiting a very high aspect ratio. Because of this, an electric field tends to be concentrated at a distal end portion of this carbon nanotube and hence this carbon nanotube is expected to exhibit high electron-emitting capability. Further, since this carbon nanotube is characterized by high chemical and physical stability, the adsorption or reaction of residual gases in vacuum unlikely occurs during the operation thereof. Therefore, this carbon nanotube is characterized in that it is resistive to ion bombardment and also to exothermic damages that may result from the emission of electrons.
If the carbon nanotube is to be utilized as an emitter, there is a known method for forming the emitter, wherein the carbon nanotube is processed to form a paste, which is then coated on the surface of a substrate by means of printing method. For example, JP-A 2003-272517 discloses a method of forming an emitter by means of screen printing method. First of all, cathodes are formed into a pattern of stripe with a predetermined pitch and then a paste containing carbon nanotube is applied by means of screen printing method over the pattern of cathodes at the same pitch as that of the cathode pattern so as to form isolated carbon nanotube-containing regions each having a rectangular or circular configuration. Then, by means of screen printing method, an insulating layer is formed between the carbon nanotube-containing resin regions. The resultant layers are subsequently baked in an air atmosphere, thereby decomposing the resin component in the carbon nanotube-containing paste layers. As a result, the carbon nanotube is exposed, thereby forming electrode-emitting portions. Finally, a grid electrode is formed over the insulating layer to manufacture the emitter.
The paste to be used in the manufacture of the aforementioned emitter is generally prepared by adding a solvent, a dispersing agent, glass frit acting as an adhesive, a filler, etc., to carbon nanotube powder to form a mixture, and then blending the mixture so as to disperse the carbon nanotube powder in the mixture form a uniform distribution of these components. After finishing the blending, the resultant mixture is subjected to filtration to prepare a paste having the carbon nanotube mixed in a vehicle consisting of the solvent and a resin. Then, the resultant paste is thoroughly agitated to enhance the dispersed state of the paste and then subjected to filtration to prepare a carbon nanotube paste. Thereafter, the surface of a substrate is printed with the carbon nanotube paste obtained from the aforementioned process and then subjected to drying and baking treatments, thereby oxidizing and decomposing the vehicle to obtain a carbon nanotube film. Although it is possible to create a carbon nanotube film on the surface of cathodes by making use of the aforementioned method, it is difficult to orientate the carbon nanotube film relative to the surface of the substrate. Further, in the case of the field-emission-type electron-emitting element, it is preferable to form a nanocarbon material at a higher orientation relative to a substrate and to increase the aspect ratio of a nanocarbon material or of a substrate provided with a nanocarbon material, thereby enhancing the electric field concentration efficiency and enabling the emission of electrons at a lower voltage.